The present invention relates to an erosion protection material, and more particularly, to a low dielectric constant erosion resistant component for the leading edge of a rotor blade.
Rotor blade assemblies, and in particular the rotor blades thereof, must accommodate various dynamic loads such as bending loads, both flapwise (out-of-plane) and chordwise (in-plane), axial loads (centrifugal), and torsional loads (pitch). Such dynamic loads subject the rotor blade to varying degrees of stresses and strains.
In addition to the operational loads, the rotor blades are also subjected to a wide variety of environmental conditions. Experience has shown that rain and/or sand particles that impinge upon the leading edges of rotating rotor blades may have an adverse effect thereon by causing erosion wear of the leading edges. Erosion wear is of particular concern at the outboard end of the rotor blades due the higher rotational velocities thereof (the rotational speed at a given span point of the rotor blade is directly proportional to the radial distance from the rotor hub) wherein the relative impact velocities of rain and/or sand particles are significantly higher.
Several different techniques have been explored to increase the erosion resistance of rotor blade leading edges. One known technique is the incorporation of ductile metal leading edge cap or abrasion strip as an integral part of the rotor blade. To further reduce erosion of the leading edge caps, it is also known to apply a sacrificial material over the leading edge caps. Typically, the sacrificial material is an elastomeric material that may be applied as a tape. Another known technique is the incorporation of a ceramic leading edge cap as an integral part of the rotor blade.
These known leading edge cap techniques accommodate the dynamic stress environment of a rotor blade during operation while providing wear resistance protection against the wide variety of environmental conditions encountered during flight operations. However, each of these known techniques includes components which may have a relatively high Radar Cross Section (RCS) and may be a primary cause of radar reflections. Even known highly erosion resistant ceramic materials based on silicon nitride and yttrium SiAlON, have dielectric constants (DK) of 7.9 or higher. A high RCS may be undesirable in many environments.
Those skilled in the art of passive RCS reduction will recognize that it would be highly preferable to have rotor blade leading edge components with lower dielectric constants while providing the erosion benefits that ceramics have over metals such as nickel or titanium. Accordingly, it is desirable to provide enhanced erosion wear protection components which accommodate the high stress environment of a rotor blade, while providing a low dielectric constant.
Erosion resistant ceramic components according to the instant invention are preferably synthesized from a silicon oxynitride precursor powder with a sintering aide. The sintering aide, preferably aluminum oxide and/or lutetium oxide are added to the silicon oxynitride precursor powder and ball milled in distilled, deionized water or other solvents. The composition is then decanted and dried to evaporate volatiles. Once dried, the material is granulated such as by passage through a 20-mesh screen.
After granulation, the material is formed into the shape of the desired component by hot pressing in a graphite die. Preferably, a low pressure on the order of 3.5 Mpa is applied to the die during hot pressing during heat up to approximately 1400xc2x0 C. followed by a ramp to the final pressing pressure to improve die longevity.
A heat treatment process may also be applied after hot pressing to further increase erosion resistance. The previously hot pressed component is placed inside a boron nitride enclosure and heat-treated for 2 to 24 hours under one atmosphere of nitrogen at 1500xc2x0 C.
The heat treating crystallizes amorphous grain boundary phases which result in a material having sand erosion characteristics superior to an electro-formed nickel component commonly used for rotor blade erosion strips. The erosion resistant ceramic component further exhibits a dielectric constant which is much lower than electro-formed nickel component.
The erosion resistant ceramic components according to the instant invention are particularly applicable to main rotor blades. In addition, leading edge erosion protection of tail rotor blades, engine propellers, turbine blades, various turbomachinery components and other components where high levels of erosion protection, and a low dielectric constant are desired will also benefit from the instant invention.